The invention is particularly suitable for use in fuel assemblies for pressurized water reactors comprising a support structure having two end fittings interconnected by guide tubes and having structural grids distributed along the assembly, with each structural grid having at least two mutually crossed sets of crossed plates and outer belt delimiting cells, some for receiving the guide tubes and the others for receiving fuel rods, the plates being provided with means for limiting the clearance for the rods about the nodes of a regular array and being fixed to at least some of the guide tubes, at least some of the grids (those placed downstream in the flow direction) being provided with mixing fins for mixing the streams of coolant.
At least one of the structural grids also serves to support the rods. To this end, it is usually provided with springs that may be cut out in the plates or added thereto, serving to urge the rods against bosses stamped in the plates forming the other faces of each cell.
Often, support is provided by the top or the bottom structural grid. The other structural grids do not participate in providing support, i.e., they provide practically no contribution to holding the rods vertically. However, some of them at least often have the function of holding the rods in a plane, i.e., centering them accurately. To this end, each face of each rod-receiving cell in a structural grid carries a boss for bearing against the rod. The bosses project by such an amount that the rod has a sliding fit in the cell.
To increase heat exchange between the rods and the coolant and to make temperatures more uniform at each level, it has already been proposed to insert an additional mixing grid in one or more of the gaps between pairs of structural grids.
In particular, proposals have been made to use lightweight mixing grids provided with fins and having respective belts, and to place these grids between the conventional structural grids in the top portion of the assembly (when the coolant flows through the core upwards, as is generally the case).
However, because two additional grids of two adjacent fuel assemblies may strike or rub against each other, the belt must be strong. To make it strong, it is made up of outer plates that are thicker and/or higher than the internal plates, thereby increasing neutron absorption and headloss.
An attempt has been made to solve this problem (see U.S. Pat. No. 4,827,063) by using additional grids without belts, constituted by plates that are entirely flat and fixed to the guide tubes to delimit cells of a size such as to enable the rods to pass through them with clearance.
Such additional grids suffer from drawbacks. They place practically no limits on the clearance available for rods which may be subject to bending vibration, particularly in reactors whose thermal power is increased by increasing the flow rate and thus the flow velocity of the coolant. Such vibration leads to the coolant-stirring fins of the additional grid being hammered.